Gear-shift device of manual transmission

ABSTRACT

A gear-shifting device for a manual transmission, in which the operational force applied at the change-lever L in the shift operation is transmitted selectively to actuate a synchro-sleeve for a gear shift, comprises a shift arm  53 , which is rotatable in correspondence to the shift operation of the change-lever L, and a 1st-2nd speed shift piece  41 , which is in contact with the shift arm  53  and is capable of shifting in response to the rotation of the shift arm  53 . In the gear-shifting device, the shift arm  53  has heteromorphous cams at its contacting part, which is in contact with the 1st-2nd speed shift piece  41 , and the heteromorphous cams are designed in a compound arc figure, which comprises a plurality of combined arcs having different curvature radii. As a result, while the shift arm  53  is rotating in correspondence to the shift operation, the distance between the contacting part and the rotational axis of the shift arm  53  varies to change the leverage effective between the change-lever L and the contacting part.

FIELD OF THE INVENTION

The present invention relates generally to a gear-shifting device for amanual transmission, which device actuates a shift fork to bring intoengagement selectively a clutch that establishes a power transmissionthrough a corresponding gear train in gear-shifting control. The presentinvention relates particularly to a gear-shifting device in which aforce applied in operating the change-lever manually is transmitted to ashift fork to bring a corresponding clutch into engagement in the manualtransmission.

BACKGROUND OF THE INVENTION

Generally, in a manual transmission (manual gear-shift transmission),when the driver operates the change-lever manually, the force applied bythe driver in the operation is transmitted through a gear-shiftingdevice to a shift fork, so that the shift fork sets a correspondingclutch (for example, a synchromesh mechanism) into engagement,establishing a desired speed change ratio for the transmission. Suchclutches are arranged over either of the two parallel shafts of thetransmission, in correspondence to permanently meshing gear trains ofvarious gear ratios, which gear trains are disposed over the twoparallel shafts in parallel with one another.

Such a manual transmission comprises a system (gear-shifting device)that transmits the operational force (shifting force) applied by thedriver at the change-lever. For example, Japanese Laid-Open PatentPublication No. 2003-14114 discloses such a system that comprises ashift selector shaft, a shift arm and a plurality of shift pieces. Inthis case, the shift selector shaft, which is equipped with the shiftarm, is movable in its axial direction and rotatable axially incorrespondence to the operation of the change-lever, which is connectedthrough connecting members like a shift cable. Shift forks are providedin the same number as the gear trains that are used as the speed changeratios of the transmission, and each shift fork has, as a one-piecebody, a shift fork shaft, on which a corresponding shift fork isprovided. When the driver operates the change-lever appropriately, theshift arm is shifted in the axial direction of the shift selector shaftand engages selectively with one of the shift pieces, and then the shiftarm is rotated around the shift selector shaft to shift thecorresponding shift fork shaft in its longitudinal axial direction. As aresult, the operational force applied at the change-lever is transmittedselectively to the corresponding one of the shift forks for a gearshift.

Into such a clutch mechanism, a synchromesh mechanism is adopted toachieve a smooth clutching operation for a gear shift, which iscontrolled by the operation of the change-lever. In this case, it isdesirable to lighten the burden of the driver in operating thechange-lever, so there are a plurality of methods of reducing the forcerequired for operating the change-lever and for transmitting thisreduced operational force securely to the shift fork to achieve a gearshift. For example, there is a method for increasing the leverage orstroke of the change-lever. Another method tries to increase thecapacity of the synchromesh mechanism to reduce the load generated inthe clutch during the synchronization.

However, there is a limit to the increase of the leverage that iseffected by increasing the stroke of the change-lever because theinstallation space of the change-lever in the interior of a vehicle islimited. Also, it is not desirable from the point of view of thedriver's maneuverability of the change-lever. Moreover, if the leverageis increased, then the stroke of the shift fork must be reduced ininverse proportion. This may result in an impairment to the secureclutching operation. On the other hand, the increasing of thesynchronization capacity results in an increase in the number of theparts constituting the transmission and can lead to an increase in themanufacturing cost of the transmission.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a gear-shiftingdevice for a manual transmission, which device has a variable leverageto improve the maneuverability of the change-lever, which has anappropriate and secure stroke for a smooth shift operation.

A gear-shifting device for a manual transmission according to thepresent invention is to transmit the operational force applied at thechange-lever in a shift operation so as to actuate selectively asynchro-sleeve for a gear shift. The gear-shifting device comprises ashift arm, which is rotatable in correspondence to the shift operationof the change-lever, and a shift piece, which is in contact with theshift arm and is capable of shifting in response to the rotation of theshift arm. Furthermore, the shift arm has heteromorphous cams at itscontacting part, which is in contact with the shift-piece. While theshift arm is rotating in correspondence to the shift operation, thedistance between the contacting part and the rotational axis of theshift arm varies to change the leverage effective between thechange-lever and the contacting part.

For the above described gear-shifting device, it is preferable that themanual transmission comprise a plurality of speed-change gears and asynchromesh mechanism, which synchronizes a synchro-sleeve and aspeed-change gear by pushing the synchro-sleeve onto the speed-changegear. In addition, preferably, the gear-shifting device is designed suchthat the leverage becomes maximum at the time of the synchronization bythe synchromesh mechanism.

Furthermore, in the gear-shifting device, the heteromorphous camspreferably have a compound arc figure which comprises a plurality ofcombined arcs having different curvature radii. By this arrangement, theleverage can be made to change after the synchronization when thecontacting part of the shift arm transits from the surface defined byone arc to that defined by another arc among these arcs in response tothe further rotation of the shift arm.

Because the top part of the shift arm, which engages with the shiftpiece to shift a corresponding shift fork shaft with a shift fork, isprovided with the compound arc figure, which comprises a plurality ofcombined arcs having different curvature radii, the leverage effectivebetween the change-lever and the shift fork is changeable while theshift arm is being rotated (and the shift fork is being shifted) by theoperation of the change-lever.

Therefore, in the initial stage of the operation of the change-lever,the leverage is set at a relatively large value to make a relativelysmall operational force applied by the driver act on the shift fork as alarge force to ensure the synchronization of the synchromesh mechanism.On the other hand, this large force acting on the shift fork is nolonger necessary after the synchronization, so the leverage is designedto change to a lower value, halfway through the shift operation executedby the driver. In this way, the rate of the displacement of the shiftfork to that of the change-lever is increased, halfway through theoperation, for the complete actuation of the shift fork. Therefore, thestroke of the change-lever for the completion of the shift operation inthe gear-shifting device according to the present invention is stillmade equal to that of a prior-art gear-shifting device, so the presentinvention does not require an increase in the installation space of thechange-lever especially in the interior of the vehicle.

In the above described gear-shifting device, the point where theleverage changes from a larger value to a smaller value during the shiftoperation can be set to come after the synchronization of thesynchromesh mechanism. In this way, the rate of the displacement of theshift fork to that of the change-lever is increased after thesynchronization to quickly bring the gear into engagement. As a result,so-called “two-step engaging load”, which is a reaction generated at thetime of the gear meshing in the synchromesh mechanism and which isuncomfortable to the driver who is operating the change-lever, isreduced in frequency and in magnitude.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram describing patterns for the operation of achange-lever.

FIG. 2 is a schematic diagram describing a system that transmits theoperational force applied on the change-lever.

FIG. 3 is a sectional plan view showing components of the system thattransmits the operational force in a manual transmission equipped with agear-shifting device according to the present invention.

FIG. 4 is a view showing parts of the gear-shifting device and partssurrounding the device.

FIGS. 5A, 5B, 5C and 5D are step-by-step views showing chronologicallythe rotation of the shift arm during the shift operation, which arm isprovided in the gear-shifting device.

FIG. 6 is a graph describing a relation between the stroke of thesynchro-sleeve and that of the change-lever, which strokes are observedduring the shift operation in the gear-shifting device according to thepresent invention.

FIG. 7 is a graph describing a relation between the stroke of thesynchro-sleeve and that of the change-lever, which strokes are observedduring the shift operation in a prior-art gear-shifting device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now, a preferred embodiment of gear-shifting device for a manualtransmission according to the present invention is described inreference to FIGS. 1˜7.

This manual transmission, when its Low, 2nd˜5th or Reverse clutch isselectively engaged, transmits the driving force of an engineselectively through a gear train with a different speed change ratio.For a gear shift, the driver manually operates the change-lever Lprovided at the driver seat. The following is a description of themechanism to achieve a gear shift.

The change-lever L, which is used to select one of the 1st˜5th speedchange ratios and reverse ratio (hereinafter referred to as “R speed”),is operated in the patterns shown in FIG. 1. If the change-lever L isoperated in the direction indicated by SE in the drawing, then it can bepositioned at one of the three selecting positions: 1st-2nd speedselecting position X1, 3rd-4th speed selecting position X2, and 5th-Rspeed selecting position X3. Then, while the change-lever L is at the1st-2nd speed selecting position X1, if it is shifted in the directionindicated by SH, which is perpendicular to the above mentionedoperational direction SE, either the 1st speed position LOW or the 2ndspeed position 2ND is selectable. Likewise, while the change-lever L isat the 3rd-4th speed selecting position X2, if it is shifted in thedirection indicated by SH, either the 3rd speed position 3RD or the 4thspeed position 4TH is selectable. Also, at the 5th-R speed selectingposition X3, if the change-lever L is shifted in the direction indicatedby SH, either the 5th speed position 5TH or the reverse position R isselectable.

When the driver operates the change-lever L to any of the abovementioned speed change positions, the change-lever L swings around theposition indicated by point S in FIG. 2 as fulcrum. As a result, theforce applied on the change-lever L by the driver in the operation istransmitted through a shift cable 11 to a shift arm 53, which rotatesaround its central axis A. Therefore, the system to transmit theoperational force from the change-lever L to the shift arm 53 comprisesa plurality of levers, which have leverages of L1/L2 and L3/L4,respectively, a shift arm 53 (the part indicated by L4), the abovementioned shift cable 11, and parts for connecting these components.

FIG. 3 shows the system that transmits the operational force (shiftingforce), which comprises the gear-shifting device according to thepresent invention. This operational force transmitting system 30, whichis provided in the transmission case 21, comprises a 1st-2nd speed shiftfork shaft 31, a 3rd-4th speed shift fork shaft 33 and a 5th-R speedshift fork shaft 35, which are movable longitudinally and axially. The1st-2nd speed shift fork shaft 31 has detent grooves 31 a, whichconstitute a detent mechanism 32. By this mechanism, the 1st-2nd speedshift fork shaft 31 is positioned at any one of the three positions,LOW, Neutral, and 2ND. Likewise, the 3rd-4th speed shift fork shaft 33and the 5th-R speed shift fork shaft 35 have detent grooves 33 a and 35a, respectively, which constitute detent mechanisms 34 and 36,respectively. By these mechanisms, the 3rd-4th speed shift fork shaft 33is positioned at any one of the three positions, 3RD, Neutral, and 4TH,and the 5th-R speed shift fork shaft 35 at either of the two positions,5TH and Reverse (R), respectively.

Furthermore, the 1st-2nd speed shift fork shaft 31, the 3rd-4th speedshift fork shaft 33 and the 5th-R speed shift fork shaft 35 areconnected to a 1st-2nd speed shift piece 41, a 3rd-4th speed shift piece43 and a 5th-R speed shift piece 45, respectively, which are supportedin the transmission case 21. These shift pieces 41, 43 and 45 aremovable axially together with their respective shift fork shafts 31, 33and 35. FIG. 3 shows only the 3rd-4th speed shift piece 43 because thisdrawing shows a condition that the shift pieces 41, 43 and 45 arepositioned one over another in the direction perpendicular to the papercarrying the drawing.

Each shift piece 41, 43 or 45 is provided with an approximately U shapedselector groove 41 a, 43 a or 45 a, respectively, and one of theseselector grooves 41 a, 43 a and 45 a is engaged with the top part of theshift arm 53, which will be described in detail later. When thechange-lever L is swung in the SE direction shown in FIG. 1, eachselector groove 41 a, 43 a or 45 a selectively and correspondingly comesinto contact with the top part of the shift arm 53. If the change-leverL is swung in the SH direction in FIG. 1, then the shift piece 41, 43 or45 that has the selector groove 41 a, 43 a or 45 a selectively engagedwith the shift arm 53 is shifted with the corresponding shift fork shaft31, 33 or 35 in the axial direction.

The 1st-2nd speed shift fork shaft 31, the 3rd-4th speed shift forkshaft 33 and the 5th-R speed shift fork shaft 35 are provided with a1st-2nd speed shift fork 61, a 3rd-4th speed shift fork 63, and a 5th-Rspeed shift fork 65, respectively. The 1st-2nd speed shift fork 61 isengaged with a 1st-2nd speed synchro-sleeve 71, which actuates a 1stclutch or a 2nd clutch (not shown). The 3rd-4th speed shift fork 63 isengaged with a 3rd-4th speed synchro-sleeve 73, which actuates a 3rdclutch or a 4th clutch (not shown). The 5th-R speed shift fork 65 isengaged with a 5th-R speed synchro-sleeve 75, which actuates a 5thclutch or a reverse clutch (not shown).

With this arrangement, the appropriate operation of the change-lever Lmakes the shift arm 53 to engage with any of the selector grooves 41 a,43 a and 45 a of the 1st-2nd speed shift piece 41, the 3rd-4th speedshift piece 43 and the 5th-R speed shift piece 45 and then makes theengaged 1st-2nd speed shift fork shaft 31, 3rd-4th speed shift forkshaft 33 or 5th-R speed shift fork shaft 35 to shift in its axialdirection. As a result, the 1st˜5th clutches and the reverse clutch areselectively actuated to establish the corresponding speed change ratios,LOW˜R.

Now, in reference to FIG. 4, the gear-shifting device according to thepresent invention is described for a case where the operational force istransmitted from the shift arm 53 to the 1st-2nd speed shift fork 61 asan example. As shown in the drawing, the gear-shifting device 50comprises the above mentioned 1st-2nd speed shift piece 41, a shiftselector shaft 51, the above mentioned shift arm 53, and a detentmechanism 80. The shift arm 53 is provided with circular holes, throughwhich the shift selector shaft 51 is placed extending in the directionperpendicular to the paper carrying the drawing. The shift arm 53 isfixed on the shift selector shaft 51 by a bolt 54, which is screwed intoa flank of the shift selector shaft 51.

The shift selector shaft 51 shifts in the longitudinal axial direction(the direction perpendicular to the paper carrying the drawing), whenthe change-lever L is swung in the SE direction in FIG. 1 and rotatesclockwise or counterclockwise around the axis A indicated in the drawingwhen the change-lever L is swung in the SH direction in FIG. 1. In otherwords, when the change-lever L is swung in the SE direction shown inFIG. 1, the shift arm 53 is shifted in the direction perpendicular tothe paper. On the other hand, when the change-lever L is swung in the SHdirection in FIG. 1, the shift arm 53 is rotated around the axis A.

As shown in FIG. 4, the shift arm 53 at its neutral position is incontact with the 1st-2nd speed shift piece 41 by the contacting part 53c thereof. Therefore, when the shift arm 53 rotates around the axis A,the operational force is transmitted from the shift selector shaft 51through the contacting part 53 c to the 1st-2nd speed shift piece 41,shifting the 1st-2nd speed shift fork shaft 31 axially. The shift arm 53at its top part (where it engages with the 1st-2nd speed shift piece 41)has a compound arc figure (heteromorphous cams). In the neutral positionshown in FIG. 4, the arc portion 53 a (near the root of the protrusion)that is the closer of the arc portions to the shift selector shaft 51 isin contact with the shift piece 41 through the contacting part 53 c.However, if the shift arm 53 rotates around the axis A, the contactingpart 53 c of the shift arm 53 in contact with the shift piece 41 shiftsto the arc portion 53 b (at the top of the protrusion) that is thecloser of the arc portions to the 1st-2nd speed shift fork shaft 31,transmitting the operational force from the change-lever L to the shiftpiece 41. This will be described in detail later.

The top part of the shift arm 53 is defined not by partiallysuperimposing complete circles without any bias, but rather by partiallycutting arcs so that the arcs have partially flattened sections. Thesepartially flattened arcs are to maintain a clearance between the shiftarm 53 and the shift piece 41 and yet to shift the 1st-2nd speed shiftfork shaft 31 axially by a sufficient distance when the shift arm 53rotates.

Furthermore, the shift arm 53 is provided with a detent mechanism 80,which comprises a flanged retaining tube 81, a ball 82, a spring 83, anddetent grooves 84, 85 and 86. The retaining tube 81 has an axis E thatis perpendicular to the axis of the shift selector shaft 51. The ball 82is held inside the retaining tube 81 so that it can move along the axisE of the retaining tube 81. The spring 83 is also provided inside theretaining tube 81, so that it biases the ball 82 toward the shift arm53. The three detent grooves 84, 85 and 86 are providedcircumferentially on the shift arm 53 at even intervals, so each of themcan engage individually with the ball 82.

The detent mechanism 80 defines angular positions (where the ball 82enters the right or left detent groove 84 or 86, respectively) for theshift arm 53, which is turned clockwise and counterclockwise from itsneutral position (where the ball 82 is in the detent groove 85) shown inFIG. 4. When the shift arm 53 is turned around the axis A for apredetermined angle counterclockwise (until the ball 82 enters thedetent groove 86) from the neutral position, the 1st-2nd speed shiftfork shaft 31 is shifted leftward (with the 1st-2nd speed shift fork 61)in FIG. 4, engaging the 1st clutch. On the other hand, when the shiftarm 53 is turned around the axis A for a predetermined angle clockwise(until the ball 82 enters the detent groove 84) from the neutralposition, the 1st-2nd speed shift fork shaft 31 is shifted rightward(with the 1st-2nd speed shift fork 61) in FIG. 4, engaging the 2ndclutch.

When the change-lever L is operated to the 2ND position, the shift arm53 is rotated to engage the 2nd clutch. Now, the movement of the shiftarm 53 and the 1st-2nd speed shift piece 41 being effected in thisinstance is described chronologically in reference to FIGS. 5A, 5B, 5Cand 5D.

In the neutral position, as shown in FIG. 5A, the right and left arcportions 53 a of the top part of the shift arm 53, which arc portions 53a are located closer to the shift selector shaft 51, are in contact withor in proximity to the right and left walls of the approximately Ushaped selector groove 41 a of the shift piece 41, at the contactingpart 53 c of the shift arm 53.

When the driver starts to operate the change-lever L to the 2NDposition, the shift arm 53 starts to rotate clockwise around the axis Aas shown in FIG. 5B. In this instance, the operational force applied atthe change-lever L is transmitted to the shift piece 41 through thecontacting part 53 c between the shift arm 53 and the shift piece 41,shifting the shift piece 41 rightward in the drawing.

When the change-lever L is operated further toward the 2ND position,increasing the displacement of the change-lever L, the shift arm 53rotates further clockwise from the condition shown in FIG. 5B to thatshown in FIG. 5C. In this instance, the contacting part 53 c of the toppart of the shift arm 53, which part is in contact with the shift piece41, shifts to the arc portion 53 b that is the farther of the arcportions from the rotational axis A of the shift arm 53. As a result,the operational force from the change-lever L is transmitted to theshift piece 41 through this new contacting part 53 c, which pushes thewall of the shift piece 41, shifting the shift piece 41 furtherrightward in the drawing. In the drawings, line B extends radially fromthe rotational axis A of the shift arm 53 through the center of the toppart of the shift arm 53, and line C that extends through the contactingpart 53 c and line D that extends through the rotational axis A areperpendicular to line B. Here, the length L4 (that corresponds to lengthL4 in FIG. 2) defined as the distance between line C and line D islonger in the condition shown in FIG. 5C than in FIG. 5B by the distancethat the contacting part 53 c has shifted in this transition.

If the change-lever L is operated further to the 2ND position, then theshift arm 53 rotates clockwise and stops at the predetermined angularposition that is defined by the detent mechanism 80 as shown in FIG. 5D.At this point, the displacement of the change-lever L has reached itsmaximum (full stroke). Correspondingly, the 1st-2nd speed shift forkshaft 31 has shifted by the predetermined distance axially, so the1st-2nd speed shift fork 61, which is mounted on the 1st-2nd speed shiftfork shaft 31, engages with the 1st-2nd speed synchro-sleeve 71, whichengages the 2nd clutch.

Here, the leverage (lever ratio) R from the change-lever L to the shiftarm 53 for the change-lever L to rotate the shift arm 53 is described asfollows by using the lengths L1˜L4 shown in FIG. 2.R=(L1×L3)/(L2×L4)

In this equation, only length L4 is a variable, which changes during theperiod from the start to the completion of a shift operation, and theothers L1˜L3 are constants. Therefore, the leverage R varies independence only on length L4 during the shift operation. While theleverage R is relatively high, the force required for the operation ofthe change-lever L is inversely small, reducing the burden of thedriver. However, the displacement of the shift piece 41 (and thesynchro-sleeve 71) also becomes inversely small.

FIG. 6 describes how the leverage (lever ratio) R changes while theoperational force is being transmitted from the shift arm 53 to the1st-2nd speed shift fork 61 as the shift operation progresses with thedriver's operation of the change-lever L. In this graph, the horizontalaxis represents the displacement of the synchro-sleeve 71 while thevertical axis represents the displacement of the change-lever L, whichis operated by the driver to actuate the synchro-sleeve 71. The leverageR is represented by the slant line in the graph.

In the condition shown in FIG. 5B (where the arc portion 53 a is incontact), length L4 is comparatively short, so the leverage R is large.Therefore, the inclination of the line that represents the relationbetween the displacement of the synchro-sleeve 71 and that of thechange-lever L shown in FIG. 6 is comparatively large. On the otherhand, when the contacting part 53 c between the top part of the shiftarm 53 and the shift piece 41 shifts from arc portion 53 a to arcportion 53 b, the inclination of the line shown in FIG. 6 becomessmaller.

As shown in FIG. 6, the point where the change of the leverage R occursis set to come after the point of the synchronization of thesynchro-sleeve 71, which has shifted for an actuation. This point of thesynchronization indicates the position in the stroke of thesynchro-sleeve 71 where the synchromesh mechanism is actuated to pushthe corresponding gear for a gear shift, generating a friction for thesynchronization. As shown in the graph, the leverage R at thesynchronization point is comparatively large. Therefore, it is enough toapply a relatively small force at the change-lever L to achieve arelatively large force acting on the 1st-2nd speed shift fork 61 forgenerating a friction for the synchronization by the synchromeshmechanism.

After the completion of the synchronization by the synchromeshmechanism, such a large force acting on the synchro-sleeve 71 is nolonger needed, but only a comparatively small force is needed to furthershift the synchro-sleeve 71. For this reason, the leverage changes to asmall value before the further displacement of the change-lever L ismade. Because of this decreased leverage, the relatively shortdisplacement of the change-lever L thereafter for the completion of thestroke achieves a sufficient displacement of the synchro-sleeve 71 toeffect a gear shift. In this way, the stroke of the change-lever Lrequired for a gear shift in the gear-shifting device according to thepresent invention can be made equal to that of a prior-art gear-shiftingdevice.

As a contrast to what is described in FIG. 6, FIG. 7 shows a relationbetween the stroke of the synchro-sleeve and that of the change-leverobserved during the operation of the change-lever in a prior-artgear-shifting device, which is equipped with a conventional shift arm.In this case, the leverage is invariable throughout the operation of thechange-lever without any consideration of the passing of thesynchro-sleeve over the synchronization point, so the inclination of theline in the graph is constant.

The above embodiment according to the present invention is an example inwhich the top part of the shift arm is designed to change the leverageof the shift operation between two values. However, the presentinvention is not limited to this. Of course, the shift arm may bedesigned to vary the leverage among three values.

As described above, in the gear-shifting device according to the presentinvention, the top part of the shift arm, which engages with a shiftpiece to shift a corresponding shift fork shaft with a shift fork, isdesigned to have a compound arc figure, which comprises a plurality ofcombined arcs having different curvature radii. By this arrangement, theleverage of the device from the change-lever to the shift fork isvariable while the change-lever is being operated to rotate the shiftarm for shifting the shift fork.

As a result, a relatively large leverage available in the initial stageof the operation of the change-lever makes a relatively small forceapplied by the driver for a gear shift act on the shift fork as a largeforce. This ensures the synchronization of the synchromesh mechanism. Onthe other hand, during the shift operation of the driver, the leveragechanges to a lower value at the completion of the synchronization, sothe relatively short displacement of the change-lever L thereafter forthe completion of the stroke achieves a sufficient displacement for theshift fork to complete the actuation. Therefore, the stroke of thechange-lever in the gear-shifting device according to the presentinvention is still made equal to that of a prior-art gear-shiftingdevice, so the present invention does not require an increase in theinstallation space of the change-lever in the interior of the vehicle.

In addition, it is preferable that the point where the leverage changesfrom a larger value to a smaller value be set after the point of thesynchronization of the synchromesh mechanism. In this way, the rate ofthe displacement of the shift fork to that of the change-lever isincreased after the synchronization so as to quickly bring the gear intoengagement. As a result, the reaction that is generated at the time ofthe gear meshing in the synchromesh mechanism is reduced in frequencyand in magnitude. This is an advantage because the reaction is generallyreferred to as “two-step engaging load”, which is uncomfortable to thedriver, who is operating the change-lever.

1. A gear-shifting device for a manual transmission in which anoperational force applied at a change-lever for a shift operation istransmitted selectively to actuate a synchro-sleeve for a gear shift;wherein: said gear-shifting device comprises a shift arm, which isrotatable in correspondence to said shift operation of saidchange-lever, and a shift piece, which is in contact with said shift armand is capable of shifting in response to said rotation of said shiftarm; said shift arm having heteromorphous cams at a contacting partthereof, wherein said cams are in contact with said shift piece andwherein said heteromorphous cams have at least two different camprofiles which contact a same surface of said shift piece; and whilesaid shift arm is rotating in correspondence to said shift operation, adistance between said contacting part and a rotational axis of saidshift arm varies to change a leverage effective between saidchange-lever and said contacting part.
 2. The gear-shifting device for amanual transmission, as set forth in claim 1, wherein: while saidchange-lever is being operated from a neutral position to a gearedposition, said leverage becomes smaller halfway through the operation.3. The gear-shifting device as set forth in claim 1 or 2, wherein: saidshift piece is provided with an approximately U shaped selector groove;and said contacting part of said shift arm is fitted in said selectorgroove.
 4. The gear-shifting device as set forth in claim 1 or 2,wherein: said shift arm is mounted on a shift selector shaft, which isrotated in correspondence to the shift operation of said change-lever.5. The gear-shifting device for a manual transmission, as set forth inclaim 1 or 2, wherein: said manual transmission comprises a plurality ofspeed-change gears and a synchromesh mechanism, which synchronizes saidsynchro-sleeve and one of said speed-change gears by pushing saidsynchro-sleeve onto said speed-change gear; and said leverage is maximumat a time of synchronization by said synchromesh mechanism.
 6. Thegear-shifting device for a manual transmission, as set forth in claim 5,wherein: said heteromorphous cams have a compound arc figure, whichcomprises a plurality of combined arcs having different curvature radii;and said leverage changes to a smaller value when said contacting parttransits from a surface defined by one arc to a surface defined byanother arc among said arcs in response to the rotation of said shiftarm after the synchronization.